1. Field of Invention
The present invention relates to a high-strength stainless steel sheet, and particularly relates to a high-strength stainless steel sheet for civil engineering and construction structural materials.
2. Description of Related Art
Conventionally, as high-strength stainless steel sheets for structural materials of which corrosion resistance is required, cold-rolled austenitic stainless steel sheets, or martensitic stainless steel sheets, which have been tempered and annealed, have been widely used.
However, austenitic stainless steel sheets have a low Young's modulus, which is disadvantageous when it comes to ensuring rigidity in structural design. Also, austenitic stainless steel sheets may exhibit structural defects because of the strains introduced during cold rolling, and further, the costs of manufacturing austenitic stainless steel sheets are high because approximately 8% by mass of Ni, which is expensive, is used. Moreover, martensitic stainless steel sheets exhibit poor ductility, and markedly deteriorated workability.
On the other hand, ferritic stainless steel sheets have good ductility, but exhibit a low strength. Attempts have been made to improve the strength of ferritic stainless steel sheets by cold-rolling to increase strength, but this method reduces ductility because of the introduction of rolling strain, and there have been cases of fracturing at the time of forming.
An attempt has been made to deal with these problems by using a mixed structure of ferrite and martensite, thereby establishing both high strength and high ductility. For example, Japanese Examined Patent Application Publication No. 7-100822 (Japanese Unexamined Patent Application Publication No. 63-169334) discloses a method for manufacturing a high ductility and high strength chrome stainless steel strip with small in-plane anisotropy. In this method, a steel slab containing 10.0% to 14.0% of Cr, 3.0% or less of Ni, and 3.0% or less of Cu, and satisfying the following conditions:C+N=0.01 to 0.12%andNi+(Mn+Cu)/3=0.5 to 3.0The steel slab is subjected to hot rolling, then cold rolling two or more times, with intermediate annealing therebetween and continuous finishing heat treatment, which consists in heating to a two-phase region temperature (α+γ region) of ferrite+austenite, which is the Ac1 point or higher but 1,100° C. or lower, and then cooling to 100° C. at a cooling rate of 1 to 500° C. per second.
Also, Japanese Examined Patent Application Publication No. 7-107178 (Japanese Unexamined Patent Application Publication No. 63-169331) discloses a method for manufacturing a high strength chrome stainless steel strip with superb ductility. In this method, a steel slab containing 10.0% to 20.0% of Cr, 4.0% or less of Ni, and 4.0% or less of Cu, and satisfying the following conditions:C+N=0.01 to 0.20%andNi+(Mn+Cu)/3=0.5 to 5.0The stainless steel strip is subjected to hot rolling, cold rolling one time without intermediate annealing, and continuous finishing heat treatment, which consists in heating to a two-phase region temperature (α+γ region) of ferrite+austenite, which is the Ac1 point or higher but 1,100° C. or lower, and then cooling to 100° C. at a cooling rate of 1 to 500° C. per second.
Further, Japanese Examined Patent Application Publication No. 8-14004 (Japanese Unexamined Patent Application Publication No. 1-172524) discloses a method for manufacturing a high-strength chrome stainless steel strip with superb ductility. In this method, a steel slab containing 10.0% to 20.0% of Cr, 4.0% or less of Ni, and 4.0% or less of Cu and more than 1.0% but 2.5% or less of Mo, and satisfyg the following conditions:C+N=0.010 to 0.20%andNi+(Mn+Cu)/3=5.0 or lessThe stainless steel strip is subjected to hot rolling, cold rolling and continuous finishing heat treatment, which consists in heating to a two-phase region temperature (α+γ region) of ferrite+austenite, which is the Ac1 point or higher but 1,100° C. or lower, and then cooling to 100° C. at a cooling rate of 1 to 500° C. per second.
Also, conventionally, ferritic stainless steel plates such as SUS430, SUS430LX, etc., having 16 to 18% of Cr have been used for steel sheets for bicycle rims, primarily because of their good corrosion resistance. Recently, the trend is for reduced weight in bicycles, and there is a demand for reduction in the thickness of bicycle rims, so there is a need to further improve the strength of SUS430, SUS430LX, etc. (450 to 550 MPa). Normally, bicycle rims are manufactured by bending a steel sheet, overlapping the widthwise center and the widthwise ends and seam welding, then cutting to a predetermined length, forming a ring shape, and performing flash butt welding at the abutted cut ends as shown in a cross-sectional diagram (FIG. 5A) taken along line VB-VB. Accordingly, strength, toughness, and corrosion resistance are required at the weld zones.
In light of such problems, a high-strength Cr-containing stainless steel used for bicycle wheel rims is proposed in, for example, Japanese Examined Patent Application Publication No.7-51737 (Japanese Unexamined Patent Application Publication No. 1-55363), wherein the chemical composition is adjusted to 11% to 17% of Cr, 0.8 to 3.0% of Ni, and 0.05 to 0.35% of Nb, 0.05 to 0.8% of Cu, and satisfying the following conditions:C+N<0.05%Nb/(C+N)=2.5 to 7anda CRE value of 5 to 20.
This composition exhibits little material deterioration even after welding two or more times, and exhibits a proof stress of 60 kgf/mm2 (588 MPa) or more in application to bicycle wheel rims.
However, while the steel sheets (steel strips) described in Japanese Examined Patent Application Publication No. 7-100822 (Japanese Unexamined Patent Application Publication No. 63-169334), Japanese Examined Patent Application Publication No.7-107178 (Japanese Unexamined Patent Application Publication No. 63-169331), and Japanese Examined Patent Application Publication No. 8-14004 (Japanese Unexamined Patent Application Publication No. 1-55363) exhibit sufficient workability in ductility and press forming, a problem remains in that sufficient bending properties are not obtained, which is an important feature in working structural materials. Moreover, the toughness of the welding zones is insufficient.
Also, while the steel sheets (steel strips) described in Japanese Examined Patent Application Publication No. 7-51737 (Japanese Unexamined Patent Application Publication No. 1-55363), Japanese Examined Patent Application Publication No. 7-100822 (Japanese Unexamined Patent Application Publication No. 63-169334), Japanese Examined Patent Application Publication No. 7-107178 (Japanese Unexamined Patent Application Publication No. 63-169331), and Japanese Examined Patent Application Publication No. 8-14004 (Japanese Unexamined Patent Application Publication No. 1-55363) each achieve a high enough strength to contribute to the reduction in the weight of bicycles. The process of manufacturing bicycle rims includes the essential process of punching holes for spokes through the seam weld zones as shown in FIG. 5A-5C, and rims manufactured using the steel sheets (steel strips) manufactured with the techniques described in these four documents generally exhibit cracking at the seam welding zones at the time of punching the spoke holes. Thus, the techniques described in these documents present problems regarding punching workability of the weld zones.
On the other hand, cold-rolling austenite stainless steels, such as SUS304, to increase strength of bicycle rims might be conceived, but it should be noted that austenite stainless steels have a low Young's modulus, is very disadvantageous regarding rim rigidity, and manufacturing costs are high due to the use of 8% by mass or more of expensive Ni.